Seasonal Trends of Atmospheric Ice Nucleating Particles Over Tokyo

Ice nucleating particles (INPs) originating from Asia are expected to have large impacts on aerosol-cloud-precipitation interactions on local, regional, and global scales. However, their seasonal variability is poorly understood. Here, we present a year-round record of atmospheric INPs measured on Tokyo Skytree, which is the world's tallest broadcasting tower located in the Tokyo Metropolitan area. The INP number concentrations showed relatively small variations in the temperature regime below -20 degrees C, whereas the values were episodically enhanced by long-range transported Asian dusts. On the other hand, the INP spectra in the temperature regime warmer than -20 degrees C exhibited measurable seasonal variations. Notably, the INP number concentrations in the temperature regime between -15 degrees C and -10 degrees C tended to indicate higher values in warm/wet seasons and lower values in cold/dry seasons. Our results suggest that Asian dust events and seasonal variations in certain particles of biological origin linked to local/regional meteorology might influence the seasonal trends of the INP spectra over the Tokyo Metropolitan area.